1. Field of the Invention
This invention pertains to high performance materials having superior thermal and/or acoustic insulative properties. More particularly, this invention relates to low-density thermal and acoustic insulation which can withstand elevated temperatures while retaining its insulative properties. In addition, this invention concerns insulation material suitable for use in aviation. Other aspects of the invention involve methods for manufacturing such insulation.
A modern airplane has a layer of insulation located just inside the plane""s exterior skin for the purpose of limiting the flow of heat into and out of the plane""s cabin. Since the temperature at the cruising altitude of commercial jets may be xe2x88x9230xc2x0, while the temperature in the cabin is approximately 70xc2x0, the resulting 100xc2x0 temperature gradient would, unless thermal insulation is used, lead to a significant loss of heat from the cabin.
Insulation also serves to reduce the noise level in the cabin, such noise being produced both by the plane""s engine(s) and the plane""s motion through the air.
Typically, the insulation used in planes is composed of singular or multiple layers of finely spun fiberglass blankets of various densities designed for thermal and acoustic protection, the latter against both high frequency sounds from jet engine noise as well as structural-borne lower frequency sounds. This material is very fine in fiber diameter and tends to fracture easily.
Conventional aircraft insulation has a number of shortcomings. As highlighted by several recent incidents involving the suspected failure of aircraft insulation, the most problematic of these shortcomings is the material""s performance in fires. At elevated temperatures, which may typically approach 2000xc2x0 F., conventional aircraft interior materials, including insulation, because of the materials from which it is made, begins emitting substantial quantities of thick, toxic smoke. Carbon monoxide and hydrogen cyanide are the two principal toxic combustion gases. Most cabin furnishings contain carbon and will generate both carbon monoxide and carbon dioxide when burned. Burning wool, silk and many nitrogen-containing synthetics will produce the more toxic hydrogen cyanide gas. Irritant gases such as hydrogen chloride and acrolein, are generated from burning wire insulation and some other cabin materials. Generally, carbon dioxide levels increase and oxygen concentrations decrease during fires. Although fire is a great danger, it has been determined that the toxic smoke produced by the smoldering insulation and interior materials is a grave threat in its own right. The blinding smoke will interfere with the evacuating passengers"" finding the plane""s emergency exits, and because it is toxic, it may asphyxiate passengers who do not escape quickly. More people could be killed through asphyxiation by toxic smoke than might die in the fire itself.
Recent incidents involving the suspected failure of aircraft insulation confirm the need for safer, more thermally-stable insulation. In October of 1998, the Federal Aviation Administration (FAA), responding to the crash of a Swissair flight near Halifax, Nova Scotia, a month earlier, recommended the replacement of the insulation in nearly all of the world""s 12,000 passenger jet planes. The FAA has also warned that the Mylar insulation used in passenger planes can catch fire when exposed to electrical shorts, and so the FAA has established new flammability standards for airplane insulation that require materials to withstand higher temperatures for extended periods of time.
One approach to improving aircraft insulation""s performance is to provide the insulation with a protective outer layer. The FAA has investigated xe2x80x9chardeningxe2x80x9d aircraft fuselages to increase the time it takes flames outside an aircraft to burn through the plane""s fuselage. One xe2x80x9chardeningxe2x80x9d technique under investigation involves using heat-stabilized, oxidized polyacrylonitrile fiber (PAN), which may double the time it takes flames to penetrate into the plane""s cabin. Barrier materials, such as those utilizing PAN, are composed of a random fiber mat or felt used in conjunction with existing fiberglass systems for improved fuselage burnthrough times.
Incidentally, this xe2x80x9chardeningxe2x80x9d approach is similar to that described in U.S. Pat. No. 5,578,368. The ""368 patent describes a material for use in sleeping bags having a protective outer layer made from aramid fiber, and the patent says this aramid layer imparts fire-resistance.
Accordingly, there is a real need for aircraft insulation which is able to sustain high temperatures without burning, smoking, degrading or outgassing. It is also desirable that when such insulation finally burns, it does so in a self-extinguishing manner.
xe2x80x9cLow-performancexe2x80x9d insulation commonly used in building construction for wall and ceiling barriers, as well as pipe wrappings, and even in aerospace applications such as aircraft thermal blankets, is typically made from a lightweight batting of glass fibers held together by a thermoset phenolic resin binder. This insulation material, commonly referred to as xe2x80x9cfiberglass insulationxe2x80x9d, is inexpensive and may be suitable as a low temperature thermal insulator and sound absorbing material. Such insulation has a number of serious shortcomings.
For example, fiberglass insulation is brittle in nature, meaning that when it is handled, airborne glass particles are produced. Those working with the fiberglass insulation may inhale the airborne glass particles, irritating their lungs. Glass particles may lodge in the workers"" skin, also causing irritation. Although those handling the fiberglass insulation can protect themselves by using respiratory masks and wearing protective gear, that results in added expense and inconvenience.
Another shortcoming of fiberglass insulation is that the material is hydrophilic, meaning water can permeate into and be absorbed by the insulation. The absorbed water decreases the insulation""s thermal and acoustic properties, and also increases the insulation""s weight, which is a serious problem if the insulation is used in aviation. Since airplane insulation is mounted against the plane""s skin, the insulation becomes quite cold when the plane is in flight. When warm, moist air, such as the air in the cabin, passes over the insulation, the water in that air condenses on and collects in the cold insulation. Over time, the insulation may become soggy, reducing its insulating abilities, and heavy, increasing the plane""s operating costs. While it may be possible to reduce water absorption by treating the fiberglass insulation or providing a barrier layer, this complicates the manufacturing process and makes the insulation more expensive.
Accordingly, there is a need for alternative insulative materials which have superior thermal and acoustic properties, without the inherent disadvantages of conventional insulation.
2. Description of the Related Art
It is generally known to provide composite materials, typically, textiles or filtration members, in which non-thermoplastic materials, for example, aramid fibers, are combined with thermoplastic materials, for example, polyphenylene fibers. A variety of such composite materials are discussed in U.S. Pat. Nos. 4,502,364, 4,840,838, 5,049,435, 5,160,485, 5,194,322, 5,316,834, 5,433,998, 5,529,826, and 5,753,001.
Blending of non-thermoplastic fibers with thermoplastic fibers to form consolidated composite materials is discussed in U.S. Pat. Nos. 4,195,112 and 4,780,59. The structures described in these patents are meant to serve as high density composite materials, and are intended to be used as load bearing and structural panels or as shape retaining moldable forms. It is important in considering these compositions to note that the disclosed structures are quite dense and fully consolidated, with nearly fiber-to-fiber contact and high shear loading. These structures have nearly saturated fiber to resin matrix interfaces, contributing to the high strength of these materials.
The binding of fiber blends may employ the use of low temperature sheath core technology. Such binder fibers are known as bicomponent fibers. Bicomponent fiber technology is discussed in U.S. Pat. Nos. 4,732,809 and 5,372,885. Bicomponent staple fibers have a low melting temperature sheath surrounding a higher melting temperature core, and are designed to sinter adjacent fibers upon softening, as disclosed in U.S. Pat. Nos. 4,129,675 and 5,607,531. The ""531 patent notes that the materials to be coated include aramid or polyphenylene sulfide fibers, and that coating materials which can be applied include polyphenylene sulfide.
Binding of fibers may also be accomplished using powder or pellets dispersed into a fibrous web to bind adjacent fibers. Powders may be applied through the use of carrier emulsions as well as spray or static charges to adhere the powder to the matrix fibers. Processing materials to achieve an even distribution of thermoplastic powder within the web is difficult and does not permit sufficient consolidation of melted material around adjacent fibers to serve as a structural node or junction. The use of powders as a binder in fibrous webs is discussed in U.S. Pat. Nos. 4,745,024 and 5,006,483.
Having recognized the need for high-performance insulation, the inventor has conducted a detailed investigation into the fabrication of insulation components, and insulation constructions which provide improved fuselage burnthrough performance.
Based upon this investigation, materials have been developed which offer superior thermal and acoustic performance that matches the current material""s light weight, yet does not shed airborne fibrous particles like fiberglass insulation. Furthermore, such material is inherently fire retardant, and the thermal and acoustic properties of the material can be tailored to specific applications by varying the diameter and density of the fibers used therein.
More specifically, the present invention employs high-performance component materials which are, because of their fire retardancy and low toxicity, particularly suited for use in aerospace insulation applications. The combination of such high performance materials in the current invention has produced insulation possessing unexpected thermal acoustic and physical properties not available in conventional insulating materials.
In addition to being well-suited for aerospace applications, this invention can also be used as fire retardant building insulation, high temperature insulation for pipe wrapping, fireman""s turnout gear, padding material, high temperature gasketing or filter media.
In contrast to known bicomponent binder fibers, powder binders and composite materials, the present invention relies on melting of a thermoplastic material to encapsulate adjacent non-thermoplastic fibers. The encapsulated fibers create strong structural junctions responsible for the materials"" exceptional resiliency. Also the functional maximum temperature of known bicomponent fibers does not extend to the high temperatures at which the present invention can be used.
It is accordingly an object of the present invention to provide a material that is well-suited for advanced aerospace and high temperature insulating applications, especially for use in thermal and acoustic blankets for commercial aircraft.
A further object of the present invention is to provide an insulating material incorporating a fireblocking materials within the body or as an ablative layer which addresses the FAA""s desire for the development of improved fuselage burnthrough materials for commercial aircraft.
A further object of this invention is to provide an insulating material having a mass of fibers, which fibers include a non-thermoplastic material; and nodes of thermoplastic material. The nodes at least partially surround and link portions of at least some of the adjoining fibers.
Another aspect of this invention concerns a method of manufacturing insulating material. This is accomplished by providing fibers of non-thermoplastic material, providing a thermoplastic material, and mixing the non-thermoplastic and thermoplastic materials together to obtain a fiber mix. The fiber mix is heated so that at least some of the thermoplastic material melts and forms globules which at least partially enclose portions of the non-thermoplastic fibers, and then the fiber mix is cooled so that the melted thermoplastic material globules form nodes that hold the non-thermoplastic fibers together.